A centralized metadata distributed filesystem (CMDFS) is a filesystem for storing data across many data servers in a distributed fashion and using a centralized metadata server that manages the filesystem. Examples include the Google File System and the Hadoop Distributed File System. When a client wishes to access data of the CMDFS, the client sends a request to the metadata server, which responds by sending metadata to the client indicating a data server from which to obtain the data. The client is then able to communicate directly with the appropriate data server to obtain the data.
Some systems additionally provide security for a CMDFS by allowing the metadata server to determine if the client has permission to access the files it is looking for and then providing a security ticket, which the client is able to use to access the data. A single ticket is used for any data block of a file—even if the block is replicated across several data servers, the same ticket is used to allow access to any of the replicated blocks.